Drill collars



March 11, 1969 R. A. MALOTT 3,431,950

DRILL COLLARS Filed Aug. 22, 1966 INVENTOR:

RAYMOND A. MA LOTT HIS ATTORNEY United States Patent Office 3,431,950 Patented Mar. 11, 1969 3,431,950, DRILL COLLARS Raymond A. Malott, Fullerton, Calif, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Aug. 22, 1966, Ser. No. 573,962

US. Cl. 138-177 Claims Int. Cl. F161 9/20 This invention is directed to the earth boring art and relates to apparatus for the prevention of formation pressure differential sticking of a drill string. More specifically, this invention relates to the apparatus of relieving the local pressure differential which sometimes occurs across a borehole wall and tends to hold a section of drill string to said wall and prevent its removal therefrom.

The problem of stuck pipe has long plagued the drilling industry. In relatively recent years it has been discovered that a drilling string becomes stuck in the well bore due to a fluid pressure differential that is created between a porous earth formation penetrated by the well bore and the drilling fluid present in said well. At the particular depth involved, the drilling fluid may be under a pressure head of several thousand pounds per square inch of pressure. When a porous formation is penetrated by the drill string, the drilling filtrate seeks to escape from the borehole under this pressure into the pores of the said formation, thereby forming a filter cake. The resultant fluid differential pressure forces the drill string against the well bore wall where it is frictionally held under the force of the pressure differential.

The prior art has resorted to many devices to alleviate the problem or free the stuck pipe but always with mixed success. One such device has been to circulate oil through the drill string and up the well bore annulus which tends to reduce the hydrostatic head of the well fluid and also reduce the frictional force between the pipe and the well bore.

Another prior art device, described in US. Patent No. 2,999,552, is a drill string having spiral grooves or flutes out along the length of the outer surface of the respective pipe sections. The purpose of such flutes is to equalize the pressure on the bore wall side of the drill string and hence prevent the establishment of a pressure differential across the thickness thereof. A disadvantage of this device is the machining expense to fabricate such pipe sections. Other disadvantages are incomplete coverage in the slip, tong, and tool joint area, as well as loss of weight, loss of stiffness, and loss of efficiency due to wear.

Experience has shown that it is the collar portion of the drill string that is most susceptible to pressure differential sticking due to the diameter thereof relative to the borehole diameter. The drill pipe connected above the collars, having a smaller diameter, presents a smaller area for the hydrostatic head to act upon. Furthermore, the greater diameter difference between the well bore and drill pipe presents a smaller area of tangency between the two. It is this area of tangency that constitutes the effective area of force acted upon by the well fluid pressure. The drill collars, on the other hand, have a diameter only slightly smaller than the well bore and hence present a relatively large area of tangency. Consequently, it is the drill collars that usually become stuck in the Well due to hydrostatic pressure difierential.

It is therefore an object of this invention to provide apparatus which equalizes the hydrostatic pressure within the area of tangency between the stuck collars of a drill string and the well bore.

Another object of this invention is to provide means in drill collars for reducing the frictional forces between the drill collars and the well bore wall in the event of high pressure differentials occurring between the two.

Still another object of this invention is to provide drill collars with simple and inexpensive means for assuring an equal distribution of hydrostatic pressure around the periphery thereof and over the full length of the drill collar.

A further object of this invention is to provide a local application of grease between the drill string collars and the well bore wall when the pressure differential across the collars becomes sufliciently great.

The invention and the advantages thereof will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a schematic elevation of a well bore and drill string;

FIGURE 2 is a plan section of the drill string shown in FIGURE 1 taken along line 22;

FIGURE 3 is an enlarged detail of a single section of drill collar constructed in accordance with the teachings of this invention; and

FIGURE 4 is a top end view of the drill collar shown in FIGURE 3.

The schematic of FIGURE 1 illustrates a rotary drilling rig 10 having a derrick, draw works, rotary power transmission means and mud pumps. Depending from the rig 10 is the drill string 11 extending down through the well bore 12 cut in the earth 13.

The drill string 11 is comprised of a plural number of pipes, compositely termed as the drill pipe and singularly termed as a joint of pipe, joined at their ends in series. Below the drill pipe there is connected an additional series of pipes known as drill collars 16. Below the drill collars is connected the drill bit 15. Connector means in the form of threads 18 and 19 are formed at opposite ends of the collar (FIGURE 2).

As the well depth is extended from hundreds to thousands of feet, the drill pipe begins to assume the character of a long and very flexible torque shaft in its purpose of transmitting rotary power to the bit 15 at the well bottom. Because of the continuous stress reversals during rotation it is advisable, for reasons of durability, to keep the drill pipe in tension. However, since the drill bit 15 is fundamentally a rock-crushing mechanism, there must be some compressive stress within the drill string to provide a suflicient force load on the bit. This latter function is served by the drill collars 16 which are pipe sections of unusually large annular section to achieve a maximum of weight per foot of length within the allowable circumference.

Ideally, then, the drill collars 16 are in compressive stress to provide weight on the bit 15 while most of the drill pipe is in tension.

To provide for the removal of bits and fragments of rock and soil from the well bore bottom a liquid called drilling mud is pumped down the inner bore 17 of the drill pipe. The mud flows through the bit 15 and onto the cutting surfaces thereof to constantly clean, lubricate and cool the bit. After flowing out of the drill string bore and across the bit face, the mud then flows back up the annular space between the well bore wall and the outside diameter of the drill string carrying the rock cuttings with it.

To improve the chip-carrying capacity of the mud, certain gelling substances such as bentonite are added to the mud to thicken'it and better hold the chips in suspension as it flows back to the surface. The gel also deposits along the walls of the well bore in the form of a clay-like liner to seal the well bore against the loss of mud by filtration into porous rock formations.

Another function served by the drilling mud is to provide a source of pressure to counterbalance possible blowout of any highly pressurized subsurface fluid reservoirs which may be penetrated by the drill string. For example, a drilling mud having a pressure-depth increment of 0.5

3 p.s.i./ft. will exert a pressure of 2500 psi. on the walls of the well bore at 5000 ft. of depth.

Consider, then, the event of penetration by the drill string 11 at great depth of a very porous formation having a very low internal pressure. The water and less viscous substances of the drilling mud will escape from the well annulus, through the mud cake lining on the well bore wall and into the porous formation. The velocity of such fluid movement will create a pressure differential across the well bore wall. When the drill collars 16 come into contact with said wall, as is constantly occurring due to the flexibility of the drill string, the mud cake liner is squeezed aside and forms a seal between the wall and that portion of the drill collar periphery in close proximity therewith. The resulting condition is as illustrated by FIGURE 2 showing a drill collar 16 eccentrically aligned in the well bore 12 with a mud cake seal packed within an area of tangency 21. Within the well bore there is the high mud pressure forcing the drill collar 16 into frictional contact with the bore wall and the mud cake.

In order to break this pressure differential force against the drill collars, the drill collars of this invention have a series of conduits or holes 20 drilled chordally through the wall thereof substantially transverse to or at an acute angle with the axis of the collar (FIGURE 3). Each hole is preferably longitudinally and angularly displaced from those adjacent, both above and below.

Although no particular pattern is necessary for the alignment of holes 20, aside from the criterion that the holes must not penetrate the bore 17 and the preference that the holes be angularly directed to a perpendicularly transverse plane to the collar axis, it should be understood that regular or helical patterns around the collar wall and along the length of the collar are also contemplated. That is to say, the holes 20 may be drilled through the wall between points that lie in the locus of a helix scribed around the outer surface of the collar. Furthermore, the locus of the terminal end points of the holes 20 may be a single or multiple lead helix.

Although FIGURE 4 shows the holes 20 to be aligned in four longitudinal planes passing through the length of the collar annulus, it should be understood that any convenient number of alignment planes may be used. In other words, the embodiment of FIGURES 4 and 5 shows four holes 20 distributed around 360 circumferential degrees of the collar 16, each drilled at a 90-degree angle with the longitudinal plane passing through the hole above and below and longitudinally directed along a predetermined pitch angle with the drill collar axis. However, if desired,

there may be three, five or even more chordally drilled holes 20 around the circumference of the collar between two parallel and axially aligned holes.

Each hole is preferably filled with grease having a viscosity such that the grease begins to flow from the hole when it is subjected at well temperature to a presure differential of 5 p.s.i., for example, between the opposite ends thereof. The particular grease and viscosity will depend upon the hole 20 diameter and well temperature. Materials other than grease which melt at normal well temperatures may be used as means to normally close the conduits 20 and act as a plug or closure valve therein.

Referring again to FIGURE 2, it will be seen that when a drill collar, constructed as described herein, is subjected to the condition shown, the pressure differential between the well bore and the porous formation 14 will cause the grease to flow from the holes 20, which have one end terminating within the area of tangency, and onto the interface between the mud cake and the drill collar. The presence of this grease will reduce the surface friction between the collar 16 and the well bore wall, thereby allowing the drill string to be turned or vibrated free from the stuck condition.

Sufiicient grease will be displaced from the holes 20, thereby allowing the high pressure drilling mud to flow through said holes and equalize the pressure within the area of tangency. The pressure differential being relieved, there will be no holding force to keep the drill collar pressed into frictional contact with the bore wall, consequently freeing same again for rotation or removal.

Although the description of this invention has been directed to specially constructed drill collars, it should be understood that the principles disclosed herein may also be applied to drill pipe.

I claim as my invention:

1. A section of pipe for use in a well drilling string, said section comprising:

a longitudinally extending tubular wall having an outer periphery and an inner bore;

a plurality of conduit means extending substantially transversely across and entirely within said tubular wall, each of said conduit means having opposite terminal ends that are communicative with said outer periphery;

each of said conduit means being scaled from communication with said bore.

2. An apparatus as described by claim 1 wherein said conduit means are holes passing generally chordally across and through said tubular wall.

3. An apparatus as described by claim 2 wherein said conduit means are filled with a viscous grease.

4. An apparatus as described by claim 3 wherein said holes are angularly distributed around said periphery.

5. An apparatus as described by claim 4 wherein said holes are longitudinally displaced, each as to the others.

6. An apparatus as described by claim 5 wherein said holes are angularly directed to as to a plane passing perpendicularly transverse to the axis of said tubular wall.

7. An apparatus as described by claim 6 wherein said holes are distributed throughout said tubular wall in a generally helical pattern.

8. An apparatus as described by claim 7 having three holes in said tubular wall per helical revolution.

9. An apparatus as described by claim 7 having more than three holes in said tubular wall per helical revolution.

10. A drill collar adapted to be connected into a drill string above a drill bit for use in well drilling operations, said collar comprising:

an elongated thick-walled tubular section of pipe having an open-ended axial bore therethrough;

connector means at each end of said tubular section for connecting it into a drill string;

a plurality of conduits extending through the thick wall of said tubular section in a direction substantially transverse to the bore of said section;

a number of said conduits being displaced from each other axially along said tubular section;

each of said conduits having the terminal ends thereof displaced circumferentially one from the other and in communication with the space outside said tubular section; and means normally closing at least some of said transverse conduits.

References Cited UNITED STATES PATENTS 2,676,820 4/1954 Boice 138l77 2,999,552 9/1961 Fox 138177 3,125,173 3/1964 Fox 138177 3,146,611 9/1964 Fox -320 PATRICK D. LAWSON, Primary Examiner.

U.S. Cl. X.R. 175-320 

1. A SECTION OF PIPE FOR USE IN A WELL DRILLING STRING, SAID SECTION COMPRISING: A LONGITUDINALLY EXTENDING TUBULAR WALL HAVING AN OUTER PERIPHERY AND AN INNER BORE; A PLURALITY OF CONDUIT MEANS EXTENDING SUBSTANTIALLY TRANSVERSELY ACROSS AND ENTIRELY WITHIN SAID TUBULAR WALL, EACH OF SAID CONDUIT MEANS HAVING OPPOSITE TERMINAL ENDS THAT ARE COMMUNICATIVE WITH SAID OUTER PERIPHERY; EACH OF SAID CONDUIT MEANS BEING SEALED FROM COMMUNICATION WITH SAID BORE. 